The electrocardiogram (ECG) or electrogram (EGM) of the cardiac cycle detected across sense electrode pairs located on the patient's skin or in the patient's body, respectively, is a repetitive waveform characterized by a periodic PQRST electrical activation sequence of the upper and lower heart chambers. The PQRST sequence is associated with the sequential depolarization and contraction of the atria followed by the depolarization and contraction of the ventricles, and successive PQRST complexes are separated by a baseline or isoelectric region. The PQRST electrical activation sequence commences with the P-wave indicative of the depolarization and contraction of the atria and is followed by the QRS complex indicative of the depolarization and contraction of the ventricles. The T-wave at the termination of the ST segment time delay is associated with repolarization of the ventricles. The PQRST electrical activation sequence with intact A-V activation detected across a sense electrode pair is fairly predictable in shape. The P-wave, R-wave and T-wave events occurring in sequence in the range of normal heart rates are usually readily recognized by visual examination of the external ECG or an EGM recorded by implanted electrodes that are correctly oriented with the depolarization waves. The P-wave and R-wave are readily sensed by sense amplifiers of a monitor or therapy delivery device coupled with appropriately placed sense electrode pairs.
The ECG and EGM signals are often plagued by baseline wander involving large amplitude, low-frequency, non-physiological signals that, in the IMD context, can saturate a measurement system, resulting in the loss of signal fidelity. Sources of baseline wander include patient movement that disturb the electrode-tissue interface, causing a low frequency signal to be superimposed on the EGM, and voltage after-potentials that linger in the electrode-tissue interface following delivery of pacing or cardioversion energy through the electrodes. The fidelity of recording, displaying, and analyzing ECGs is reduced by such noise. Also, in most current IMDs, the analog EGM is applied to an analog-to-digital converter (ADC) so that the encoded data can be stored in IMD memory and/or uplink telemetry transmitted. Baseline wander increases the dynamic range of the EGM signal such that higher bit resolution and higher dynamic range or the ADC are needed to digitize the analog EGM. The consumption of current from the IMD battery undesirably increases as ADC bit resolution and dynamic range are increased.
Due to the above-mentioned problems, the ECG and EGM signals must be bandpass filtered to block such noise without itself reducing ECG and EGM signal quality. The sense amplifiers of external ECG equipment are normally employed with skin electrodes applied to the patient's skin, and the ECG signals are often plagued by interference from stochastic or random noise or by 50 Hz or 60 Hz electrical mains noise in the patient's body. The fidelity of recording, displaying, and analyzing ECGs is reduced by such noise. Therefore, the ECG signals must be bandpass filtered to block such noise without itself reducing ECG signal quality. The high pass filter cut-off frequency must be selected to still enable passage of the low frequency components of the ECG, e.g., any baseline deviations due to ST segment elevation or depression caused by cardiac ischemia.
The Ad Hoc Writing Group of the Committee on Electrocardiography and Cardiac Electrophysiology of the Council on Clinical Cardiology of the American Heart Association and the Association for the Advancement of Medical Instrumentation (AAMI) recommend that the low frequency cut-off should be on the order of 0.67 Hz provided that the filter meets certain phase and amplitude requirements. Although these recommendations were written specifically for surface ECG instruments and similar documents for intracardiac electrogram instruments do not exist, they provide general guidelines for improving the value and performance of IMD based EGM recording to observe ischemia. The AAMI standards require recording instruments to reproduce a 3 mV by 100 ms step input with no more than a 100 μV overshoot. Any first order RC high pass filter with corner frequency at or below 0.05 Hz and any linear-phase high pass filter with corner frequency at or below 0.67 Hz will pass this step input test. Both of these implementations can be found in today's commercial surface ECG instruments.
Various techniques have been employed to reduce baseline distortion in ECG signals. The most common technique presently used is Cubic-Spline interpolation where the onset of QRS signals are isolated and interpolation techniques are employed to determine the smoothest curve joining several QRS onsets. The ECG baseline can then be estimated by subtracting the curve representing the baseline wander as estimated by the Cubic-Spline technique from the recorded ECG signal. However, the QRS onset points used by the Cubic-Spline method, do not sufficiently model noise in an ECG baseline due to the scarcity of interpolation points at low heart rates. Determination of the QRS onset can be difficult or impossible in the presence of high amplitude baseline wander. Furthermore, it is necessary to delay the ECG in order to obtain reference points (QRS onset points) for calculation of the Cubic-Spline curve. This delay (often as long as 5 seconds or longer) renders the Cubic-Spline interpolation unsuitable for real time monitoring of patients.
A faster method of correcting baseline distortion has been to use high pass filtering techniques. One such filtering technique is to employ Finite Impulse Response (FIR) filters as suggested in U.S. Pat. No. 5,772,603, for example. In one approach, the output is taken after the FIR transfer is combined with a delay based on an integral number of the filter. These filters offer a linear phase response providing an undistorted ECG signal. FIR filters have the disadvantage of being highly computational. Nearly ideal filter characteristics may be obtained at the price of having to perform many floating point calculations and additions.
A commonly used FIR filter is the Comb Filter which is a recursive implementation of a non-recursive FIR filter. While offering the advantage of significantly lower computational requirements, this filter offers only moderate attenuation of baseline noise. The maximum stop band attenuation of the Comb Filter is a modest −13.5 db. Another disadvantage of this type of filter is that it has a wide pass band to stop band transition in the frequency domain resulting in poor noise attenuation near the cut-off frequency.
It has also been proposed to employ high pass Infinite Impulse Response (IIR) filters which are recursive filters that offer the advantage of fewer computations than FIR filters. However, the nonlinear phase response of high pass IIR filters causes an unacceptable level of phase distortion of the ECG signal. The nonlinear phase response causes different frequency components of the signal to be delayed or shifted in time by different degrees. The worst distortion occurs at of just above the cut-off frequency of the filter for the class of filters appropriate to ECG filtering, that is, Butterworth filters that have maximally flat frequency response.
A solution to the phase distortion problem is to apply the ECG signal in a forward direction, which can be in real time, and then in a backward or reverse directions to the IIR filter, which is referred to as a “zero phase IIR filter”. The ECG baseline distortions are filtered out when the ECG passes through the zero phase IIR filter in the forward direction, but the ECG signal itself is distorted. The reverse direction filtering process corrects the distortion of the ECG signal and further attenuates baseline noise.
The use of such a zero phase IIR filter effective in achieving 0.67 Hz passband frequencies on ECG signals is disclosed in U.S. Pat. Nos. 5,297,557, 5,402,795, and 5,479,922 wherein the ECG signal is filtered in the forward direction over a selected window, and then the same window is filtered in a reverse direction to remove phase distortion. It is asserted that the delays associated with use of the zero phase IIR filter in this manner are small enough to enable real time ECG measurement and display of patients engaged in stress exercise. Two basic filter variations are disclosed that are implementations of analog 3 pole or 5 pole Butterworth filters that offer a trade-off between computational complexity, stop band attenuation, phase response and transient settling time. Other filters, such as Bessel filters, may be used in place of the design employing the Butterworth filter. The Bessel filter has the advantage of providing a design containing less overshoot in response to impulse signals and the disadvantage of providing less attenuation in the stop band.
As already mentioned, in most current IMDs, the analog EGM is applied to an analog-to-digital converter (ADC) so that the encoded data can be stored in IMD memory and/or uplink telemetry transmitted. Baseline wander increases the dynamic range of the EGM signal such that higher bit resolution and higher dynamic range or the ADC is needed to digitize the analog EGM. The consumption of current from the IMD battery undesirably increases as ADC bit resolution and dynamic range is increased. The previously mentioned methods involving use of the Cubic-Spline interpolation technique, FIR, or the zero-phase IIR filters used in '557, '795, '922 patents are digital signal processing techniques for removing baseline wander. In most of these techniques, the removal of baseline wander can only occur after the EGM to be filtered is digitized. Therefore, these techniques need to be employed with an ADC with higher bit resolution and wider dynamic range.
In commonly assigned U.S. Pat. No. 6,317,625, a signal measuring system for an IMD is provided for sensing EGM signals having a relatively large effective dynamic range due to baseline wander without increasing ADC bit resolution and dynamic range. Low frequency compression/enhancement techniques are combined with dither techniques to effectively increase the dynamic range while maintaining resolution without increasing the number of bits of the ADC that is used to convert the sensed signal to digital format. In one embodiment, the system includes a high-pass filter (HPF), an analog-to-digital converter (ADC), a decimation filter (DF), and a compensation filter (CF). The EGM (including baseline wander imposed on the EGM) is passed through the HPF, and the HPF attenuates the low frequency components of the signal. Unlike conventional systems, the HPF serves to attenuate the bias current signal so that the sampled signal remains within the dynamic range of the system. In one embodiment, the HPF attenuates frequency components that are within the frequency bandwidth of the desired output signal. The ADC then over-samples the output signal of the HPF. The DF receives the output samples of the ADC and generates output samples at a rate that is at least twice the maximum frequency of the desired output signal. The CF then amplifies the low frequency end of the DF output samples. The gain and cut-off frequency of the CF are, ideally, set to substantially offset the attenuation of the HFP for those low frequency components of the input signal below the cut-off frequency of the HPF and above the minimum frequency of the desired output signal. Although it would appear that the resolution of these low frequency components has been degraded, dither techniques are used, in effect, to exchange sample rate for resolution. In one embodiment, system noise (noise inherent in the system due to imperfections in the components, thermal noise, etc.) is used as the dither. As a result of the compression/enhancement and dither techniques, the output signal remains within the dynamic range of the system with the desired resolution, which allows the system to display an accurately measured signal significantly faster than conventional systems.
Thus, the system disclosed in the '625 patent avoids the necessity of increasing the ADC bit resolution so that the EGM can be digitized by an ADC with lower dynamic range and then reconstructs the EGM after it has been digitized. But, the reconstruction process reconstructs both the desired EGM signal and the noise or baseline wander.
Although the above-described methods and apparatus represents a substantial improvement over the prior art, further improvement is, of course, generally desirable. Thus, there is a need for a low-cost, energy-efficient, physiological signal measuring system for use in an IMD having a relatively large dynamic range, high resolution, high fidelity, and good noise rejection characteristics. The system should minimize the changes in the morphology of the physiological signal being measured and minimize the noise content of the signal recorded so that the ability to provide accurate patient diagnoses based on the signal characteristics is not only uncompromised, but is enhanced.